WO2015185956A1

WO2015185956A1 - High strength multiphase galvanized steel sheet, production method and use

Info

Publication number: WO2015185956A1
Application number: PCT/IB2014/000991
Authority: WO
Inventors: Fan DONGWEI; Jo Jun HYUN; John A. ROTOLE
Original assignee: ArcelorMittal Investigación y Desarrollo, S.L.
Priority date: 2014-06-06
Filing date: 2014-06-06
Publication date: 2015-12-10
Also published as: KR102389648B1; JP6599902B2; RU2675025C2; EP3152336B1; KR20170015303A; CA2951215A1; ES2729870T3; RU2016147787A3; JP2017520681A; CA2951215C; US20170137906A1; MX2016016129A; US11047020B2; RU2016147787A; EP3152336A1; HUE044866T2; US10612107B2; TR201907448T4; CN106471147A; WO2015185975A1

Abstract

The invention deals with a steel sheet comprising, by weight percent: 0.05 ≤ C ≤ 0.15%, 2 ≤ Mn ≤ 3%, Al ≤ 0.1 %, 0.3 ≤ Si ≤ 1.5%, Nb ≤ 0.05%, N ≤ 0.02%, 0.1 ≤ Cr + Mo ≤ 1 %, 0.0001 ≤ B < 0.0025, 3.4 x N ≤ Ti < 0.5%, V ≤ 0.1 %, S ≤ 0.01 %, P ≤ 0.05% the remainder of the composition being iron and unavoidable impurities resulting from the smelting and the microstructure contains, in surface fraction: between 50 and 95 % of martensite and between 5 and 50 % of the sum of ferrite and bainite, wherein the ferrite grain size is below 10 pm. The steel according to the invention is oxidized and subsequently reduced during heating, soaking and cooling steps of the annealing.

Description

HIGH STRENGTH MULTIPHASE GALVANIZED STEEL SHEET, PRODUCTION

METHOD AND USE

The present invention relates to high-strength multiphase steels, for motor vehicles use, which have high formability properties and exhibit high resistance levels, and are intended to be used as structural members and reinforcing materials primarily for motor vehicles. It also deals with the method of producing the high formability multiphase steel.

As the use of high strength steels increases in automotive applications, there is a growing demand for steels of increased strength without sacrificing formability. Growing demands for weight saving and safety requirement motivate intensive elaborations of new concepts of automotive steels that can achieve higher ductility simultaneously with higher strength in comparison with the existing Advanced High Strength Steels (AHSS).

Thus, several families of steels like the ones mentioned below offering various strength levels have been proposed.

Among those concepts, steels with micro-alloying elements whose hardening is obtained simultaneously by precipitation and by refinement of the grain size have been developed. The development of such High Strength Low Alloyed (HSLA) steels has been followed by those of higher strength called Advanced High Strength Steels which keep good levels of strength together with good cold formability. However, the tensile levels reached by these grades are generally low.

So as to answer to the demand of steels with high resistance and at the same time high formability, a lot of developments took place. However, it is well known that for high strength steels, trying to increase the ultimate tensile strength generally leads to lower ductility levels. Nevertheless, carmakers keep developing more and more complex parts that require more ductility without sacrificing the resistance requirements. In addition, an improvement in yield strength and hole expansion performance over steels currently in production is needed, for instance for hot dip coated steel sheets.

The invention is directed to a method of manufacturing high strength hot dip coated steel, its production method and the use of said high strength steel to produce 5 a part of a vehicle.

The US application US2013008570 is known, such application deals with an ultra high strength steel plate with at least 1 100MPa of tensile strength that has both an excellent strength-stretch balance and excellent bending workability, and a method ίθ for producing the same. The metal structure of the steel plate has martensite, and the soft phases of bainitic ferrite and polygonal ferrite. The area of the aforementioned martensite constitutes 50% or more, the area of the aforementioned bainitic ferrite constitutes 15% or more, and the area of the aforementioned polygonal ferrite constitutes 5% or less (including 0%). When the circle-equivalent diameter of the

5 aforementioned soft phase is measured, the coefficient of variation (standard deviation/mean value) is less or equal to 1 .0. The ultra high strength steel plate has at least 1 100MPa of tensile strength. Such application is silent as regards to different formability issues such as hole expansion and yield strength which have important impact on in use properties.

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It is also known the application WO2012153016, dealing with a cold rolled steel which tensile strength is above 1000 MPa and uniform elongation above 12%, as well as V bendability above 90°. The chemical composition of this application comprises, in weight percent : : 0,15% < C < 0,25%, 1 ,8% < Mn < 3,0%, 1 ,2% < Si < 2%, 0% < Al < :5 0, 10%, 0% < Cr < 0,50%, 0 % < Cu < 1 %, 0 % < Ni < 1 %, 0%< S <0,005%, 0 % < P < 0,020%, Nb<0,015%, Ti<0,020%, V<0,015%, Co<1 %, N<0,008%, B<0,001 % while Mn+Ni+Cu < 3%, the remainder being Fe and inevitable impurities from the cast. The steel microstructure contains, in surface percentage, 5 to 20 % of polygonal ferrite, between 10 and 15% of residual austenite, from 5 to 15 % of martensite, balance being lath type bainite. This application requires austenite to be stabilized through the continuous annealing process.

The aim of the invention is to solve above mentioned problems, i.e bringing a hot dip coated high strength steel with simultaneously:

A tensile strength above or equal to 980 MPa, or even 1 180 MPa

A total elongation above or equal to 8%.

A hole-expansion value superior or equal to 20%, or even 40%

A yield strength value above 500 MPa, or even 780 MPa

Another aim of the invention is to provide a process for making such hot dip coated multiphase steel, while being compatible with usual continuous annealing galvanizing lines. To do so, the invention main object is a hot dip coated steel sheet comprising, by weight percent:

0.05 < C < 0.15%

2 < Mn < 3%

Al < 0.1 %

0.3 < Si < 1.5%

Nb < 0.05%

N < 0.02%

0.1 < Cr + Mo < 1 %

0.0001 < B < 0.0025

3.4 x N < Ti < 0.5%

V < 0.1 %

S < 0.01 %

P < 0.05%

the remainder of the composition being iron and unavoidable impurities resulting from the smelting and the microstructure contains, in surface fraction: between 50 and 95 % of martensite and between 5 and 50 % of the sum of ferrite and bainite, wherein the ferrite grain size is below 10 μιη.

In a preferred embodiment, the steel chemical composition has a carbon content such that, 0.09 < C < 0.14 %.

In another preferred embodiment, the steel has a manganese content such that, 2.2 < Mn < 2.7 %. In another preferred embodiment, the steel has an aluminum content such that Al < 0.05 %.

In another preferred embodiment, the steel has silicon content such that 0.6 < Si < 1 .3%.

In another preferred embodiment, the steel chemical composition has a niobium content such that, Nb < 0.03 %.

In another preferred embodiment, the steel chemical composition has a sum of chromium and molybdenum such that, 0.1 < Cr+Mo < 0.7 %.

In another preferred embodiment, the steel chemical composition has a boron content such that, 0.001 < B < 0.0022 %. In another preferred embodiment, the steel chemical composition has a titanium content such that, 3.4 x N < Ti < 0.1 %.

In a preferred embodiment, the steel presents between 5 and 30 % of ferrite surface fraction. In a preferred embodiment, the mean ferrite grain size is below 10 μιτι, preferably below 5 pm and even more preferably below 3 pm.

In another embodiment, the mean aspect ratio of the ferrite grain size is between 1 and 3.

In a preferred embodiment, the steel presents between 5 and 40 % of bainite.

Even more preferably, the hot dip coated steel of the invention has a tensile strength of at least 980 MPa or even 1180MPa, a yield strength of at least 500 MPa or even 780MPa, a total elongation of at least 8% and a hole expansion of at least 20% or even 40%.

Preferably, the steel according to the invention is galvanized or galvannealed.

The invention has also, as a second object a method for producing a high strength steel hot dip coated sheet comprising the successive following steps:

- casting a steel which composition is according to the invention as defined above so as to obtain a slab, - reheating the slab at a temperature T_reheat above 1 180 °C,

- hot rolling the reheated slab at a temperature above 800°C to obtain a hot rolled steel,

- cooling the hot rolled steel at conventional cooling rate until a coiling temperature coiiing between room temperature and 800°C, then

- coiling the hot rolled steel cooled at T_COriing,

- de-scaling the hot rolled steel

- Optionally, the hot rolled steel is annealed at a temperature T|_A above 300°C during more than 20 minutes. - Optionally, the temperature of the hot rolled steel before entering the cover should be above 400°C. The cooling rate of the hot rolled steel should be lower than or equal to 1 °C/min and higher than or equal to 0.01 °C/min.

- cold rolling the steel so as to obtain a cold rolled steel sheet, - Heating up to a temperature T_anneai between 750°C to 950°C, annealing at T_anneai for at least 30 seconds and cooling the cold rolled steel to a temperature T₀A between 440°C and 700°C, during said heating, annealing and cooling steps, the surface of the cold rolled steel is oxidized and subsequently reduced.

- Holding the cold rolled steel at T₀A for less than 180 seconds, - Hot dip coating the cold rolled steel to obtain coated cold rolled steel,

-optionally, the hot dip coated cold rolled steel is galvannealed to reach an iron content between 7% and 15% in the cold rolled steel coating.

-the hot dip coated cold rolled steel is cooled down to room temperature at a cooling rate of at least 1 °C/s.

Preferably, the coiling temperature is so that: 500°C < T_COiiing≤ 750°C.

In a preferred embodiment, the optional annealing temperature TIA is so that 500°C≤ TIA≤ 650°C for a time between 30 hours and 100 hours.

In a preferred embodiment, the annealing temperature T_anneai is so that: 775°C ≤

Tanneai≤ 825°C.

In a preferred embodiment, the oxidizing step takes place upon heating in a direct fire furnace to a depth of at least 200nm. In another preferred embodiment, the oxidizing step takes place between 500°C and 750°C

In another preferred embodiment, the reducing step takes place in a radiant tube furnace in a mixed gas atmosphere having a dew point between below 0°C.

Preferably, the hot dip coating is done in a liquid Zn alloyed bath so as to obtain a galvanized or galvannealed cold rolled hot dip high strength steel.

In another embodiment, the hot dip coating is done in a liquid Al alloyed bath so as to obtain an aluminized cold rolled high strength steel.

The steel according to the invention can be used to produce a part for a motor vehicle.

The main aspects of the invention will now be described:

Figure 1 illustrates a microstructure of the steel according to the invention with martensite in white, ferrite and bainite in black.

To obtain the steel of the invention, the chemical composition is very important as well as the production parameters so as to reach all the objectives. Following chemical composition elements are given in weight percent.

Carbon is an element used for strengthening the martensite, If the carbon content is below 0.05%, the tensile strength of 980 MPa minimum is not reached in the present invention. If carbon is higher than 0.15%, the martensite will be hard and the total elongation of 8% will not be reached in the steel of the present invention. Furthermore, carbon is strong austenite forming element. Lowering carbon contents, from 0.15 % downwards, allows having for a given annealing temperature, less austenite and enough ferrite to improve formability and reach the total elongation target. Additionally, low annealing temperatures for the steel according to the invention limits considerably ferrite grain growth; as a consequence, the final ferritic grain size is below 0 microns. This combination contributes to the great compromise of mechanical properties obtained in the steel according to the invention.

Preferably, the carbon content is so that 0.09 < C < 0.14 %.

Manganese is a hardening element. If Mn content is below 2%, the tensile strength will be lower than 980 MPa. If the Mn content is above 3%, central segregation of Mn is expected at mid thickness and this will be detrimental to In Use Properties. Preferably, the manganese content is so that 2.2 < Mn < 2.7 %.

Silicon has a strengthening effect, it improves total elongation and hole expansion ratio as well as delayed fracture resistance. If Si content is below 0.3%, total elongation will be below 8% and above mentioned properties will be impaired. If Si content is above 1.5%, the rolling loads increase too much and cold rolling process becomes difficult. Furthermore the soaking temperature will be too high, this will lead to manufacturability issues. Moreover, coatability by hot dip coating may get impaired due to silicon oxide formation on surface of the sheet. Preferably, the Si content is so that 0.6 < Si < 1 .3 for above given reasons. Aluminum, Just like titanium, can form AIN to protect boron. However, its content is limited to 0.1 % because higher Al contents, will lead to higher annealing temperatures to have the same microstructural balance all other parameters being equal. As a consequence, for cost and energy saving purposes, its content is limited to 0.1 %. Preferably, the Al content is so that Al < 0.05%. Niobium can form precipitates, which have a grain refining effect, known to increase tensile strength. In addition it improves hole expansion ratio as well as delayed fracture resistance. If Nb content is above 0.05%, ductility will be reduced and the total elongation will fall below 8%. Preferably, the Nb content is so that Nb < 0.03%.

Mo and Cr will improve hardenability and tensile strength. If the sum of these elements is below 0.1 %, a large fraction of ferrite will form in addition to the growth of pro-eutectoide ferrite grain formed during annealing and this will decrease the strength. If the sum of these elements is above 1 % in the steel of the invention, it will make the hot band hard and difficult to cold roll. Preferably the sum of these elements is so that 0.1 < Cr+Mo < 0.7%.

Titanium is added to combine with nitrogen so as to form TiN and as a consequence protect B in solid solution, if neither Ti nor Al is present, boron nitride can form. In that case, boron would not be in solid solution and play its role defined below. Additionally TiN formation improves the formability and the weldability as well as the resistance to Delayed fracture in the steel of the invention. For these reasons Ti content is at least 3.4 times the nitrogen one. Above 0.5 %, Ti will lead to higher annealing temperatures to have the same microstructural balance all other parameters being equal. As a consequence, for cost and energy saving purposes, its content is limited to 0.5%. Preferably, the Ti content is so that 3.4xN < Ti < 0.1 %.

Boron can suppress ferrite formation during the cooling step of the cold rolled band annealing. As a result, it avoids a drop in strength below 980 MPa. If the boron content is above or equal 0.0025% (25 ppm), the excess of boron will precipitate as nitride boron at austenitic grain boundaries and these will serve as nucleation sites for ferrite formation with the same tensile drop effect on mechanical properties. Below 0.0001 % (1 ppm) higher grades it terms of tensile strength are more difficult to reach. Ideally, boron must be 0.001 < B < 0.0022 % to obtain mechanical properties above 180 MPa with a minimum of 8% of total elongation. As for vanadium, if the content is above 0.1 %, vanadium will consume the carbon by forming carbides and/or nitro-carbides and this will soften the martensite. In addition, the ductility of the steel according to the invention will be impaired and fall below 8%.

As for nitrogen, if the nitrogen content is above 0.02%, boron nitrides will form and reduce the steel hardenability since low content of free boron will be available. It will also form large fraction of AIN, which is detrimental for total elongation and hole expansion ratio. As a consequence, nitrogen content is limited to 0.02% not to fall below 8% of elongation and/or 20% of hole expansion ratio.

As for phosphorus, at contents over 0.050 wt.%, phosphorus segregates along grain boundaries of steel and causes the deterioration of delayed fracture resistance and weldability of the steel sheet . The phosphorus content should therefore be limited to 0.050 wt.%.

As for sulphur, contents over 0.01 wt% lead to a large amount of non-metallic inclusions (MnS), and this causes the deterioration of delayed fracture resistance and ductility of the steel sheet. Consequently, the sulphur content should be limited to 0.01 wt%.

The balance of the steel according to the invention is made of iron and unavoidable impurities.

The method to produce the steel according to the invention implies casting steel with the chemical composition of the invention. The cast steel is reheated above 1180°C. When slab reheating temperature is below 1180°C, the steel will not be homogeneous and precipitates will not be completely dissolved. Then the slab is hot rolled, the last hot rolling pass taking place at a temperature T|_P of at least of 800°C. If T|_P is below 800°C, hot workability is reduced and cracks will appear and the rolling forces will increase.

-Cooling the steel at a typical cooling rate known per se by man skilled in the art down to the coiling temperature T_COiiing.

- coiiing must be lower than the last pass temperature T|_P °C. This temperature is preferably below 800°C. preferably, the coiling temperature is so that 500°C < T_COiiing < 750°C.

-After coiling, the hot rolled steel is de-scaled.

-Then, optionally, the hot rolled steel is annealed at a temperature above 300°C during more than 20 minutes. If the thermal treatment is done below 300°C, the forces for cold rolling will be too high and below 20 minutes the same result is obtained, the material will be too hard to be easily cold rolled. Preferably, the thermal treatment is done between 500°C and 650°C for 30 hours to 100 hours.

-Optionally, the hot rolled steel is placed under a cover, insulated if necessary, to cover one or more coils to facilitate uniform cooling of the hot rolled product.

-In a preferred embodiment, the temperature of the hot rolled steel before entering the cover should be above 400°C. The cooling rate of the steel should be lower than or equal to 1 °C/min and higher than or equal to 0.01 °C/min. If the cooling rate is higher than 1°C/min, the hot band will be too hard for following cold rolling. A cooling rate lower than O.OrC/min, would be detrimental to productivity. -Cold rolling the steel with a cold rolling ratio that will depend on final targeted thickness.

- Heating the steel up to the annealing temperature T_anneai which must be between 750°C and 950°C. - Annealing the steel at the temperature T_anneai between 750°C and 950°C during at least 30 seconds. Controlling the annealing temperature is an important feature of the process since it enables to control the initial austenite and ferrite fractions as well as their chemical composition. Below 750°C, the ferrite will not be fully recrystallized and elongation will be below 8%, while it is useless to go above 950°C for energy and cost saving reasons. Preferably, the annealing is done within 30 and 300 seconds and the temperature is preferably between 775 and 825°C.

-during this heating, annealing and cooling steps, the steel is oxidized and then reduced. This oxidation followed by reduction step is necessary so that the steel surface is suitable for hot dip coating.

-In a preferred embodiment, the reducing step takes place in a radiant tube furnace in a mixed gas atmosphere having a dew point below 0°C. -In an even preferred embodiment, the oxidizing step takes place between 500°C and 750°C for productivity reasons.

Then the cold rolled steel is cooled. -After the cooling, the steel is held at a temperature between 440°C to 700°C for less than 180 seconds. Below 440°C, a large fraction of bainite or martensite will be formed and whether the tensile strength whether the total elongation will be below the expectations of the present invention: 980 MPa and 8% respectively. Above 700°C, hot dipping issues will appear with vaporization of the melt and the reaction between melt and strip will be uncontrolled. -Then the steel is hot dip coated to obtain a coated cold rolled steel, preferably the hot dip coating is done in a bath of Zn or Zn alloy so as to obtain a galvanized cold rolled high strength steel.

-In another embodiment, the hot dip coating is done in a bath of Al or Al alloy so as to obtain an aluminized cold rolled high strength steel.

-optionally, the hot dip coated cold rolled steel is alloyed to the substrate so as to obtain a galvannealed cold rolled high strength steel.

-Then the hot dip coated cold rolled steel is cooled down to room temperature at a cooling rate of at least 1 °Cs. Ferrite in the frame of the present invention is defined by a cubic centre structure with grain size lower than 10 microns (μηη). The sum of the content of ferrite and bainite, in the frame of the invention, must be between 5 and 50 % so as to have at least 8% of total elongation; below 5% of ferrite such elongation level will not be reached. Above 50% of the sum of ferrite and bainite, the tensile strength target of 980 MPa will not be reached. Preferably, the ferrite content is between 5 and 30%. In another embodiment, the bainite content is between 5 to 40%.

In a preferred embodiment, the ferrite grain size is below 10 pm, preferably, it is below 5 pm and even more preferably it is below 3 pm. The low grain size ferrite, i.e below 10 pm, improves the yield strength. This ferrite content range with limited size is obtained thanks to the combination of low annealing temperatures, chemical composition elements such as Nb and Ti which pin ferritic grain sizes and limit their growth as well as the presence of Cr and Mo which limit ferrite formation upon cooling after the annealing.Above 10 pm, the yield strength will be too low and below the target of 500 MPa. In an even preferred embodiment, the aspect ratio of the ferrite grain size, i.e the mean values of the ratios of the length by the height of each ferrite grain is between 1 and 3. Such measures are taken on at least 3 populations of ferrite grains, samples analyzed being observed with an optical or a scanning electronic microscope at the material third thickness for homogeneity purpose. This aspect ratio of ferrite grain size improves the homogeneity of properties, if ferrite grain size are needle types, i.e above 3 or below 1 , the difference of properties between longitudinal direction and transversal direction will be too high and the material properties will be heterogeneous and too much depending on direction of strain.

Martensite is the structure formed during cooling after the soaking from the unstable austenite formed during annealing. Its content must be within the range of 50 to 95%. Less than 50% the tensile strength target of 980 MPa is not reached and above 95%, the total elongation will be below 8%.

The good hole expansion results in this invention is due to the phase fraction balance and small difference in hardness of the phases (ferrite and martensite). Abbreviations

UTS(MPa) refers to the ultimate tensile strength measured by tensile test in the transversal direction relative to the rolling direction.

YS (MPa) refers to the yield strength measured by tensile test in the transversal direction relative to the rolling direction, TEI (%) refers to the total elongation.

UTS, YS and Tel can be measured following several tests. Tests used for the examples are done according to JIS-T standard.

HE (%) refers to the hole expansion. Such test can be performed with the help of a conical punch made of a cylindrical part which diameter is 45 mm, topped by a conical part. Such punch is being positioned under the steel sheet to test and which has been previously provided with a hole of an initial diameter Do of 10 mm. The conical punch is then being moved upwards into such hole and does enlarge it until a first traversing crack appears. The final diameter D of the hole is then being measured and the hole expansion is calculated using the following relationship:

D-Do

HE 100

Do

Microstructures were observed using a SEM at the quarter thickness location, using 2% Nital etching and quantified by image analysis.

The steels according to the invention will be better understood when reading the examples below which are given not for limitation purpose as regard to the scope but as illustrations.

Semi-finished products have been produced from steel casting; the examples in table 1 are all according to the present invention. The chemical compositions of the semi-finished products, expressed in weight percent, are shown in Table 1 below.

Table 1 : chemical composition of steels which are all according to the invention (wt%)

The rest of the steel composition in Table 1 consists of iron and inevitable impurities resulting from the melting, impurity level being lower than 0.0005 but higher than 0.0001 mill.%..

Ingots of composition 1 to 8 were initially reheated and hot rolled. The hot rolled steel plates were then cold rolled and annealed. The process parameters undergone are shown hereunder:

Reheating temperature

Finishing rolling temperature (FRT): °C

Coiling temperature (CT) : °C Cold rolling applied (CR):

Soaking temperature during annealing (Soaking T) : °C

Soaking duration during annealing: seconds (Soaking t).

Over-ageing temperature range T₀A

Over ageing time tOA Coating type : Gl for galvanized at 465°C and GA for Galvannealed

The hot rolled steels of the examples have not undergone any annealing before cold rolling, this step is optional. The cold rolling ratio for all examples is between 40 and 50%. The steels 1 to 8, according to the invention have undergone the process parameters described in table 2 annealing

before cold

number reheating TP FRTfC) CT, C rolling

: 1 1230 871 620 No

2 1230 865 62 No

3 . 1230 . 874 620 . No

. ^: 4 1230 872 580 No

5 1230 865 580 No

6 1230 874 580 No

7 1230 865 580 No

8 1230 890 700 No

Table 2 process parameters from reheating to cold rolling

It can be seen that all references have undergone process parameters according to the invention and have been cold rolled with a cold rolling ratio of 50%. In table 3 below, all steels have undergone an oxidation during heating using a direct fire furnace followed by a reduction in a radiant tube furnace according to the present invention. As a consequence, the steel surfaces are suitable for receiving a GA coating at 570°C which corresponds to the alloying of the coating to the substrate. The cooling from the GA temperature down to room temperature after galvannealing has been carried out at 5°C/s.

soaking T° soaking time tOA

TOA (°C) coating type

number Invention l°C) (sec) (sec)

1 A

2 B

3 C

4 D

775 135 470-460 40 GA at 570°C

5 E

6 F

7 G

8 H

1 1

2 J

3 K

800 135 470-460 40 GA at 570°C

4 L

7 M

8 N

1 0

2 P

3 Q

4 R 825 135 470-460 40 GA at 570°C

6 S

T

8 U

1 V

2 W

3 X

4 Y

850 135 470-460 40 GA at 570"C

5 z

6 AA

7 BB

8 CC

Table 3: annealing parameters to produce hot dip coated very high strength steels according the invention

With regard to the microstructure, the mean values for examples Q and W of table 3 have the following microstructural features:

Table 4: microstructural features of examples Q and W As for the mechanical properties, the table 4 above shows the results obtained which are all according to the invention as for yield strength, tensile strength, total elongation and hole expansion. BOG stands for broken on gauge, the value has not been obtained.

Table 5: mechanical properties

The steels according to the invention present good coatability. In addition, a lot of examples show tensile strength above 980 MPa and even above 1180 Pa (see invention W). Furthermore ductility levels are also above 8% in all cases , yield strength is above 500 MPa and even above 780 MPa in some examples (see invention W) and hole expansion values are clearly above 20 % and in the best cases above 40% (see example W). The steel according to the invention can be used for automotive body in white parts for motor vehicles.

Claims

1. Steel sheet comprising, by weight percent:

0.05 < C < 0.15%

2 < Mn < 3%

Al < 0.1 %

0.3 < Si < 1.5%

Nb < 0.05%

N < 0.02%

0.1 < Cr + Mo < 1 %

0.0001 < B < 0.0025

2. Steel sheet according to claim 1 wherein 0.09 < C 0.14%.

3.4 x N < Ti < 0.5%

V < 0.1%

S < 0.01 %

P < 0.05%

the remainder of the composition being iron and unavoidable impurities resulting from the smelting and the microstructure contains, in surface fraction: between 50 and 95 % of martensite and between 5 and 50 % of the sum of ferrite and bainite, wherein the ferrite grain size is below 10 μη

4. Steel sheet according to anyone of claims 1 to 3 wherein Al < 0.05%.

5. Steel sheet according to anyone of claims 1 to 4 wherein 0.6 < Si < 1.3%.

6. Steel sheet according to anyone of claims 1 to 5 wherein Nb < 0.03%.

7. Steel sheet according to anyone of claims 1 to 6 wherein 0.1 < Cr + Mo < 0.7%.

8. Steel sheet according to anyone of claims 1 to 7 wherein 0.001 < B < 0.0022%.

9. Steel sheet according to anyone of claims 1 to 8 wherein 3.4 N < Ti < 0.1 %.

10. Steel sheet according to anyone of claims 1 to 9 wherein 5 < ferrite surface fraction < 30%.

11. Steel sheet according to anyone of claims 1 to 10 wherein 5 < bainite surface fraction < 40%.

12. Steel sheet according to anyone of claims 1 to 11 wherein the tensile strength is at least 980 MPa, the yield strength is at least 500 MPa, total elongation is at least 8% and the hole expansion is at least 20%.

13. Steel sheet according to claim 12 wherein the tensile strength is at least 1180 MPa, the yield strength is at least 780 MPa, total elongation is at least 8% and the hole expansion is at least 40%.

Steel sheet according to anyone of claims 1 to 13 wherein the steel galvanized or galvannealed.

15. Method for producing a hot dip coated steel sheet comprising successive following steps :

- casting a steel which composition is according to anyone of claims 1 to 9 so as to obtain a slab,

- reheating the slab at a temperature T_reheat above 1 180 °C,

- coiling the hot rolled steel cooled at X_Coiiing-

- de-scaling the hot rolled steel

- Optionally, the hot rolled steel is annealed at a temperature T|_A above 300°C during more than 20 minutes.

-Optionally, the temperature of the hot rolled steel before entering the cover should be above 400°C. The cooling rate of the hot rolled steel should be lower than or equal to 1 °C/min and higher than or equal to 0.01 °C/min.

- cold rolling the steel so as to obtain a cold rolled steel sheet,

- Heating up to a temperature T_anneai between 750°C to 950°C, annealing at T_anneai for at least 30 seconds and cooling the cold rolled steel to a temperature TOA between 440°C and 700°C, during said heating, annealing and cooling steps, the surface of the cold rolled steel is oxidized and subsequently reduced.

- Holding the cold rolled steel at TOA for less than 180 seconds,

-Hot dip coating the cold rolled steel to obtain coated cold rolled steel,

-optionally, the hot dip coated cold rolled steel is galvannealed to reach an iron content between 7% and 15% in the cold rolled steel coating. -the hot dip coated cold rolled steel is cooled down to room temperature at a cooling rate of at least 1 °C/s.

16- Method for producing a hot dip coated steel sheet according to claim 15 wherein

500°C < Tcoiling≤ 750°C.

17- Method for producing a hot dip coated steel sheet according to anyone of claims 15 and 16 wherein 500°C < T_(A≤ 650°C during a time between 30 hours and 100 hours.

18- Method for producing a high strength steel hot dip coated sheet according to claim 15 to 17 wherein the oxidizing step takes place upon heating in a direct fire furnace to a depth of at least 200nm.

19- Method for producing a high strength steel hot dip coated sheet according to claim 15 to 18 wherein the reducing step takes place in a radiant tube furnace in a mixed gas atmosphere having a dew point below 0°C.

20- Method for producing a high strength steel hot dip coated sheet according to claim 15 to 19 wherein the oxidizing step takes place between 500°C and 750°C.

21- Method for producing a hot dip coated steel sheet according to anyone of claims 15 to 20 wherein 775°C < T_anneai≤ 825°C.

22- Method for producing a hot dip coated steel sheet according to anyone of claims 15 to 21 wherein the hot dip coating is done in a liquid Zn bath so as to obtain a galvanized or galvannealed cold rolled high strength steel. 23- Method for producing a hot dip coated steel sheet according to anyone of claims 15 to 21 wherein the hot dip coating is done in a liquid Al bath so as to obtain an aluminized cold rolled high strength steel.

24- Use of a steel sheet according to anyone of claims 1 to 14 or produced according to anyone of claims 15 to 23 to produce a part for a motor vehicle.